The present invention relates to a sleeve for an indirect or offset printing machine and in particular to an offset blanket cylinder.
As is well known, an offset machine or a lithographic rotary machine with indirect printing mainly comprises three cylinders. A first cylinder carries lithographic plates and is in contact with inking rollers and wetting rollers. A second, subsidiary cylinder (or blanket cylinder) receives the inked data to be printed (i.e., xe2x80x9cthe impressionxe2x80x9d) from the first cylinder. These data are transferred to a substrate or web of paper or other material (for example plastic), interposed between the blanket cylinder and a third cylinder or pressing (or printing) cylinder. After transforming the inked data to the substrate, the surface of the blanket cylinder passes through a bath of solvents that wash the residual ink from the surface of the blanket cylinder.
The blanket cylinder is usually covered with a natural rubber blanket, which can have either a xe2x80x9ccompressiblexe2x80x9d structure, i.e., with a compressible layer, or a xe2x80x9cconventionalxe2x80x9d structure, i.e., without a compressible layer. Various methods (and corresponding products) for producing the blanket cylinder are known. One of these uses a blanket of flat natural rubber with a yieldable (compressible) structure. The cylinder has an axial slot disposed parallel to the longitudinal axis. The rubber is wrapped about the blanket cylinder with its ends inserted into the slot and fixed to the cylinder by inserting a bar into the axial slot to retain the ends of the rubber therein.
The use of this type of blanket cylinder gives rise to various problems. For example, the presence of said slot results in mechanical imbalance of the cylinder structure. When the slot passes through the contact region between the respective cylinders, the pressure exerted between the blanket cylinder and the printing cylinder (or plate cylinder) varies. This cyclic pressure variation leads to vibration and stresses on the blanket cylinder and results in poor print quality on the substrate.
Said imbalance also limits the maximum rotational speed of the cylinder. Exceeding the maximum rotational speed generates stresses that can mechanically damage the printing machines. This limitation in rotational speed in turn limits the amount of printed substrate that can be produced in a given amount of time.
The presence of the slot also results in wastage of the substrate by creating a void in the print on the substrate.
This known method and resultant solution was later overtaken by other solutions. For example, offset presses began using a rotary support or mandrel that carries a cylindrical blanket sleeve, which together with the mandrel function as the blanket cylinder. This blanket sleeve includes an inner cylindrical portion or core that is formed as a hollow cylindrical body or sleeve. The core is typically formed of a thin-walled nickel tube that has a radial thickness in the range of seven thousandths of an inch thick to ten thousandths of an inch. The core is configured to be selectively drawn over the mandrel and locked to the mandrel. Thus, the blanket sleeve can be mounted on and dismounted from the mandrel, as by pressurized air for example, and therefore is independent from the rotary mandrel of the offset press. The blanket sleeve includes a compressible layer positioned on the inner cylindrical portion (core), a substantially incompressible reinforcement layer positioned on the compressible layer, and finally a printing layer that receives the inked data.
The compressible layer comprises a first continuous tubular body (without joints) of elastomeric material (natural rubber) presenting internally a plurality of cavities that determine the xe2x80x9ccompressibilityxe2x80x9d of the layer. To produce this compressible layer on the inner cylinder (core) first requires placing the natural rubber material into solution to form a liquid. This is accomplished by adding solvents to the solid natural rubber to provide the rubber in liquid solution. Then microspheres (that ultimately will produce the desired cavities in the compressible layer) are mixed into that rubber solution. Then, in a very time consuming process that requires considerable operator skill, the natural rubber solution with the microspheres is applied to the surface of the inner cylinder (core) by a knife coating technology or ring coating technology for example to build up a precursor layer of about one millimeter in radial thickness. However, because natural rubber does not adhere well to nickel surfaces, when the core is formed of nickel, an adhesive preparation must be provided. For example, a two-sided adhesive tape is typically first wound around the nickel core, and the rubber solution is applied to the exposed surface of the tape rather than to the bare nickel surface.
The use of a knife coating technology to produce this precursor layer requires an operator to mount the core onto a rotating mandrel. As the mandrel rotates, the operator must apply the liquid rubber solution with the microspheres to the surface of the rotating core. At the same time, a knife blade rises automatically to even out the surface being created while heated air is applied to remove the solvent from the solution as the core is rotating. The amount of solution being applied by the operator will vary depending on the consistency of the solution. If the solution is running it will not form the solid layer around the core. The consistency of the solution depends on the atmospheric ambient conditions of temperature, humidity and barometric pressure. These conditions also affect whether the solvent is removed completely during each revolution of the core on the mandrel. The solvent, which is volatile, must be completely removed prior to the next step, which is subjecting the precursor layer to heat that is sufficient (100 to 130 degrees centigrade) to cure the rubber. The generation of the precursor layer using the knife coating technology takes on the order of two to three hours for a typical sleeve or cylinder.
Once this preliminary thickness of the precursor layer has been attained, the natural rubber forming the precursor layer must be cured by the application of heat and pressure in another time-consuming process that requires operator manipulation of the cylinder. First, a tape that shrinks when subjected to curing temperatures (noted above) is wound around the precursor layer. The taped sleeve may be placed into an oven and maintained at curing temperatures (noted above) for two to three days. As the tape shrinks, the necessary pressure is applied to the precursor layer in order to effect curing of the natural rubber. Once the curing step is done, the cylinder must be manipulated to another station where the surface of the precursor layer can be ground down to the desired thickness (typically three tenths to seven tenths of a millimeter) of the compressible layer forming a tubular body.
Reinforcement structures such as threads or meshes (of cotton or other material) can be built on top of the compressible layer. The reinforcement layer can be defined by an elastomeric matrix containing threads, preferably of cotton. The threads can be continuous or discontinuous. These reinforcement structures can be applied spirally or linearly on the compressible layer. The function of this reinforcement layer is to form a support structure with physical and mechanical characteristics that are far superior to those of the elastomeric natural rubber matrix that forms the compressible layer and the outer printing layer (now to be described).
Finally, the surface printing layer is formed of elastomeric material (natural rubber) on top of the tubular body with the reinforced structure. The surface printing layer can be formed like the compressible layer, except without the use of microspheres and the voids created thereby. Alternatively, the surface printing layer can be formed by another technology such as by extrusion of a natural rubber sleeve onto and around the reinforcing layer. The final surface of the outer printing layer is continuous and without joints. All of the layers of the known sleeve are all bonded together to form a single body. However, the required operator involvement and manipulation steps in the production process required to fabricate the known blanket sleeve prevent significant automation of this fabrication process. The low level of automation adversely affects the consistency of the sleeve that can be produced.
The consistency of the compressible layer is important for printing quality, and end users of the blanket sleeves are specifying acceptable ranges for compressibility. Moreover, the compressibility must stay within the specified range over time. However, the consistency of the compressible layer obtainable in the known rubber blanket sleeves is limited by the high degree of operator involvement and judgment during the fabrication process as well as by the unpredictable ambient conditions under which different sleeves are made for the same end-user. Moreover, residual solvent in the compressible layer will continue to create voids in the compressible layer and thus changes the compressibility of the overall sleeve over time. Residual solvent is a consequence of the fabrication process of the known rubber blanket sleeves. Thus, while a known rubber blanket sleeve may be delivered to the end-user with an acceptable compressibility, the compressibility of that sleeve may change enough over time to become outside the acceptable range.
Moreover, the aforesaid known blanket cylinder presents an outer layer of natural rubber or elastomeric material with inferior physical and mechanical characteristics, equivalent to those of rubber. The outer layer has poor mechanical strength, at least partly because of these characteristics of natural rubber. Consequently, the outer layer undergoes considerable wear during use. This wear is caused by the action on this outer layer of the blanket sleeve by the metal plate of the plate cylinder or by the edges of the substrate being printed, or by poor resistance to the wash solvents used in the printing process. A fold or other thickness variation in the substrate can irreversibly damage the surface of the outer layer and render the entire cylinder useless. Moreover, the recurring pressure applied to the printing surface during repeated printing on the press eventually overcomes the outer layer""s reboundability, i.e., its ability to resist permanent compression. Once the original thickness of this outer printing layer is diminished, the blanket sleeve becomes incapable of transferring the inked data to the substrate with the desired resolution of the printed image. This is particularly a problem in presses that print on both sides of the substrate and thus have a blanket cylinder on each side of the substrate, thus potentially doubling the problem as a bad image on one side of the substrate renders the entire substrate useless. Additionally, when the sleeve has a thin nickel core, the sleeve can become irreversibly damaged because the thin nickel core tends to kink during mounting and dismounting of the sleeve onto the rotary mandrel of the offset printing machine. These factors combine to curtail the xe2x80x9cuseful lifexe2x80x9d or duration of a blanket sleeve of the aforesaid known type. This curtailment presents obvious drawbacks from an economical viewpoint, especially in the cost of employing an offset printing machine that requires a plurality of blanket cylinders.
An object of the present invention is therefore to provide a blanket cylinder and/or blanket sleeve having superior physical and mechanical characteristics than known cylinders and/or sleeves such as to offer higher wear resistance, better reboundability, and greater resistance to creases in the surface and hence prolong the useful life of the product, said blanket sleeve being able to be removably coupled to the rotary member or support (mandrel) of the offset printing machine to form a portion of said blanket cylinder.
A further object is to provide a blanket sleeve of the stated type having a lower cost than known sleeves for known blanket cylinders.
A still further object of the invention is to provide a method whereby a blanket sleeve of the stated type can be produced in a shorter time than conventional sleeves.
A yet further object of the invention is to provide a method whereby a consistent blanket sleeve of the stated type can be produced regardless of ambient conditions and personnel available during production.
Another object of the invention is to provide a method whereby a blanket sleeve of the stated type can be produced by procedures that are more automated than the procedures for making conventional sleeves.
Additional objects and advantages of the invention will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, a blanket cylinder and method for making same now will be described in summary fashion.
A blanket cylinder is provided that employs polyurethane material for the compressible layer instead of the natural rubber found in conventional blanket sleeves. The compressible layer of the improved blanket sleeve can be provided with a density in the range of between about 0.2 g/cm3 and 0.9 g/cm3 and desirably between about 0.5 g/cm3 and 0.9 g/cm3. Polyurethane also may be used to form the incompressible blanket layer instead of the natural rubber found in conventional blanket sleeves. The incompressible blanket layer of the improved blanket sleeve can be provided with a density in the range of between about 1.0 g/cm3 and 1.6 g/cm3. In some embodiments, a reinforcing layer may be interposed between the compressible layer and the incompressible blanket layer.
To further achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, the improved method of making the improved blanket sleeve includes providing a cylindrical body to define the inner cylindrical portion of the blanket sleeve and forming at least one blanket layer including polyurethane material carried by the cylindrical body and defining a printing surface for receiving the inked data to be transferred to the substrate. The method also desirably includes providing the cylindrical body composed of nickel, or a metal wire mesh or resin embedded with fiber such as fiberglass, carbon fiber, or aramid fiber.
The method desirably includes forming a compressible layer between the blanket layer and the inner cylindrical portion by depositing a first pasty polyurethane material on the outer surface of the inner cylindrical portion and causing the first pasty polyurethane material to solidify on the outer surface of the inner cylindrical portion to define the compressible layer of the sleeve. The first pasty polyurethane material is preferably elastomeric such as a polyether polyurethane or polyester polyurethane. The first pasty polyurethane material can be obtained by mixing a polyol and microspheres having a shell of a phenolic type of thermosetting resin surrounding a gas like isobutane or by mixing a polyol and swelling agents that release gas when heated or by mixing a polyol and water-soluble salts such as sodium chloride, magnesium chloride or magnesium sulphate. Ribbon technology is desirably used for depositing the first pasty polyurethane material on the outer surface of the inner cylindrical portion. Causing the first pasty polyurethane material to solidify on the outer surface of the inner cylindrical portion is desirably accomplished by cross-linking the first polyurethane material at ambient pressure. This cross-linking can be allowed to proceed for about five hours if carried out at ambient temperature or can be accelerated by the addition of heat and/or cross-linking agents. The compressible layer can be ground to the desired thickness and uniform surface.
The method desirably includes forming the incompressible blanket layer on the compressible layer. The incompressible blanket layer can be formed of a second pasty polyurethane material that is preferably elastomeric such as a polyether polyurethane or polyester polyurethane. Alternatively, the method includes forming the incompressible blanket layer on a reinforcing layer that is formed around the compressible layer. The reinforcing layer can be formed in any conventional way. In each case, the blanket layer desirably can be formed by ribbon technology or by extrusion technology for example. If formed by ribbon technology, cross-linking can occur at ambient pressure. Cross-linking also can occur at ambient temperature or can be accelerated by the addition of heat and/or cross-linking agents. The incompressible blanket layer can be ground to the desired thickness and uniform surface.